Battery-powered devices (e.g., consumer electronic devices, electric and hybrid automobiles, etc.) are charged from a power source (e.g., AC power outlet) through a charging device. The charging device couples the battery to the power source through an adaptor. The cord extending between the power source and the battery-powered device can take up space. In situations where multiple devices require charging, each with their own charger and cord, the charging area can become cramped and inconvenient.
Approaches are being developed that use over-the-air or wireless power transmission between a transmitter and a receiver coupled to the electronic device. Wireless power transmission using inductive coils is one method considered as an un-tethered method for transferring power wirelessly through a coupled wireless power signal. In wireless power transmission, power is transferred by transmitting a wireless power signal through a transmit coil. On the receiver side, a receive coil may couple with the transmit coil through the wireless power signal, thus, receiving the transmitted power wirelessly. The distance between the transmitter coil and receive coil, at which efficient power transfer can take place, is a function of the transmitted energy, the distance, and the alignment of the power transfer coils. The coupling coefficient (k) is a function of the distance and alignment between the coils, the coil sizes, and materials. The power conversion efficiency (e.g., coupling factor, coupling quality) may be significantly improved if the coils are sized and operated at such a frequency that they are physically within the so-called “near-field zone” of each other.
An issue encountered during wireless charging transmission includes situations when the transmitter and receiver are weakly coupled, which may occur due to misalignment of the transmitter and receiver, the presence of a foreign object, or for other reasons. These issues may cause the wireless power transfer to be less efficient, which may also result in excess heat. Conventional foreign object detection methods may rely on coil temperature measurements (coils tend to heat up when foreign objects are present), or comparing the input power to output power to determine if the output power is within an expected efficiency threshold (e.g., 350 mW). These methods may fail because the efficiency estimate may be unreliable regarding for some typical operating conditions, which may result in errors in detection.